energies

Article

An Experimental Study on the Characteristics of NOxDistributions at the SNCR Inlets of a Large-Scale CFB Boiler

Jin Yan 1,2,* , Xiaofeng Lu 2, Changfei Zhang 1,3, Qianjun Li 1,3, Jinping Wang 3, Shirong Liu 4, Xiong Zheng 2

and Xuchen Fan 2

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Citation: Yan, J.; Lu, X.; Zhang, C.; Li,

Q.; Wang, J.; Liu, S.; Zheng, X.; Fan, X.

An Experimental Study on the

Characteristics of NOx Distributions

at the SNCR Inlets of a Large-Scale

CFB Boiler. Energies 2021, 14, 1267.

https://doi.org/10.3390/en14051267

Academic Editor: Artur Blaszczuk

Received: 8 February 2021

Accepted: 23 February 2021

Published: 25 February 2021

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1 College of Energy and Power Engineering, Nanjing Institute of Technology, Nanjing 211167, China;drdx1984@njit.edu.cn (C.Z.); hjliqianjun@njit.edu.cn (Q.L.)

2 Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education of PRC,Chongqing University, Chongqing 400044, China; xfluke@cqu.edu.cn (X.L.); x.zheng@cqu.edu.cn (X.Z.);20181001003@cqu.edu.cn (X.F.)

3 Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University,Nanjing 210096, China; wjp@seu.edu.cn

4 Datang Wu’an Power Plant Co. Ltd., Handan 056300, China; zjb@cdt-wafd.com* Correspondence: jinyan@njit.edu.cn; Tel.: +86-18151004185

Abstract: The unknown NOx distributions inside large-scale CFB (circulating fluidized bed) boilershave always hindered the economy of the SNCR (selective non-catalytic reduction) process. Inthis study, field tests were carried out on a typical 300 MW CFB boiler, where multi-level 316 L-made probe and Ecom-J2KN/Testo 350 analyzers were used to perform detailed two-dimensionaldistributions of flue gas composition at SNCR inlets for the first time. The penetration depth insidethe horizontal flue pass was up to 7 m. The NOx distributions were analyzed in detail combiningwith the auxiliary test in the dilute phase zone. Key results show that the average O2 concentrationsin #A and #C regions were 6.52% and 0.95%, respectively. The vertical NOx distributions of #A and#C SNCR inlets were similar, showing a trend of first increasing and then decreasing with peak valueall appeared at 5 m depth, while the NOx distribution of #B SNCR inlet was basically increasing.Some local areas with extremely high NOx concentration (over 2000 mg/m3) were observed near theinclined edge of SNCR inlets, which has never been reported before. Based on this, the optimizationof urea injections was conducted, which could save 15.7% of the urea solution consumption whileensuring ultra-low emission of NOx.

Keywords: large-scale CFB; NOx distribution; SNCR; urea optimization

1. Introduction

In recent years, the circulating fluidized bed (CFB) technology has developed rapidlybecause of its advantages in fuel flexibility and load regulation [1–4]. The low-temperaturecombustion inside the furnace also provides sufficient benefits for de-NOx combustion [5,6]and high-efficient desulfurization [7]. Meanwhile, the operating temperature of the cycloneranges from 800 to 950 ◦C, which is very suitable for the selective non-catalytic reduction(SNCR) system. The high-velocity rotation and strong turbulence intensity of flue gaspromote the mixing of NOx with reduction agents. In general, the NOx emission can belimited below 100 mg/m3 with SNCR technology [8,9]. However, with the increasinglystringent requirements of environmental protection, the conventional treatments have beendifficult to meet the updated NOx emission standard (50 mg/m3 at 6% O2) [10].

Thus, various de-NOx combustion technologies emerged as the times required, in-cluding air-staging [11], improvement of cyclone efficiency [12], fluidization state specifi-cation [13], flue gas recirculation [14] and optimization of urea injection [15]. Most utilityboilers could meet the ultra-low emission standards after retrofit, while the problems ofhigh NH3 escape and low NOx-removal efficiency existed in a large number of powerplants.

Energies 2021, 14, 1267. https://doi.org/10.3390/en14051267 https://www.mdpi.com/journal/energies

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Including but not limited to CFB boiler, many scholars focused on the mechanism ofSNCR reactions to improve de-NOx efficiency. Taking the cyclone of a 660 MW CFB boileras the object, Kang et al. [16] obtained a simplified 18-element mechanism using CHEMKINsoftware; the optimal temperature window was then proposed by combining with CFDsimulation. When the gas temperature was not sufficient, the NOx-removal efficiency couldalso be improved to a certain extent by increasing the ratio of ammonia to nitrogen (NSR).Yao et al. [17] compared the effect of CO and CH4 on NO reduction in a tubular reactorwith simulated flue gas. At low temperatures, the additive of C3H8 was the most efficientin enhancing SNCR process. At pressurized oxy-combustion condition, kinetic modelingwas carried out by Rahman et al. [18] to simulate and optimize the SNCR process. Thede-NOx efficiency increased as the pressure increased from 1 to 10 atm. At high pressure,the generation of NNH showed to be the most promoting reaction. Swieboda et al. [19]reviewed the application of SNCR technology in pulverized coal-fired boilers and pointedout that the measurement of exhaust gas parameters was the most direct validation forSNCR optimization.

It is generally believed that significant non-uniform input parameters exist in large-scale CFB boilers, including primary air and secondary air [20,21], fuel particle disper-sion [22], coal feed distribution [23,24], recirculating ash and heat absorption deviation inmulti-parallel loops. Therefore, the flue gas composition at furnace outlets should alsobe uneven. Generally speaking, the smaller the uniformity deviation of NSR distributionis, the higher NOx-removal efficiency is. On the contrary, the worse the mixing of ureasolution with NOx will be, which not only affects the NOx-removal efficiency, but also maygenerate a sharp increase of NH3 escape in some local areas. Therefore, it is particularlyimportant to obtain the actual NOx distributions at SNCR inlets for accurate urea injections.In the selective catalytic reduction (SCR) system commonly adopted in pulverized coalboiler, the non-uniform NOx distribution and flow field can be directly calculated [25] oreven measured [26] due to low solid concentration and gas temperature. For example, Liuet al. [27] proposed a prediction-assisted feed-forward to enhance outlet NOx control; 22key operating parameters were considered in the MLR (multi-layer perception) methodand the original dynamic NOx emission was predicted successfully.

However, due to stronger thermal inertia and harsher in-furnace processes, it isdifficult to dynamically monitor the NOx distributions at the SNCR inlets of CFB boilers.Thus, on-site measurement shows to be the key means to solve this problem. Niklassonet al. [28] performed on-line measurements with zirconia cell probe successfully. Hartgeet al. [29] designed novel probes to measure flue gas at various levels inside a 235 MW CFBboiler. The maximum penetration depth reached 3 m. A wall region with a thickness of 0.5m was observed in the test, and the non-uniformity of gas composition in front and rearwalls was attributed to fuel distribution. With a self-made sampling device system, theauthors once measured the gas along the horizontal direction of secondary air ports with amaximum sampling depth up to 4 m [30]. The oxygen distribution and jet penetration atthe lower part of the furnace were basically predicted.

The above literature indicates that many studies are available concerning the SCRprocess or the reaction mechanism during SNCR process, but few are related to the non-uniformity of NOx concentration at the SNCR entrance of CFB boilers. In particular,relevant field tests have rarely been reported. Nonetheless, the accumulation of the studiesin [28,29,31] inspires the scholars to further explore more detailed NOx distributions inlarge-scale CFB boilers through the field test approach. Thus, the present contributionreports a preliminary investigation into a typical 300 MW CFB boiler, focusing on measuringthe characteristics of NOx distributions at the SNCR inlets. By comparison with theauxiliary test in the dilute phase, the non-uniformity of NOx distribution was analyzed indetail. In addition, local regions with ultra-high NOx concentration were captured at theinclined edge of the SNCR inlets for the first time. Based on the measured NOx distributions,the regulation of precise urea injections was proposed, which could save 15.7% of the ureaconsumption while ensuring ultra-low emission of NOx. It is the first time to achieve

Energies 2021, 14, 1267 3 of 15

the sampling depth up to 7 m, and the detailed two-dimensional distributions of NOxconcentration were obtained. The results provide the lasting source for the improvementof CFB combustor model, and a direct reference for the prediction of NOx distribution atSNCR entrance to enhance de-NOx efficiency in large-scale CFB boilers.

2. Experimental Section2.1. The 300 MW Utility CFB Boiler

As shown in Figure 1, the tested 300 MW CFB boiler has single furnace, single airdistributor and three steam-cooled cyclones, labeled A–C from the left wall to the right wall.The cross-sectional area of the furnace is 28.3 × 9.8 m2. Circulating ash return from therear wall through six seal loops to improve the uniformity of bed temperature distribution.Eight coal feed ports are arranged on the front wall, the primary air enters the windboxfrom two sides and the secondary air is blown into the chamber through the high and lowlevels at the front and rear walls. Eighteen L-shape platen heat exchangers consist of threedifferent types with the same numbers, namely intermediate temperature superheater(ITS), high temperature superheaters (THS) and high temperature reheaters (HTR). Theyprotrude into the furnace at two heights from the front wall. Two water-cooled evaporationwalls are arranged on the rear wall. The SNCR system with urea solution injections isinstalled on each side of the three cyclone inlets. The main design parameters of this boilerare exhibited in Table 1.

Figure 1. Schematic diagram of the 300 MW CFB boiler.

Table 1. Design parameters of the 300 MW CFB boiler.

Item Unit Value (BMCR)

Superheat steam flow rate t/h 1100Superheated steam pressure MPa 17.4

Superheated steam temperature ◦C 541Reheat steam pressure MPa 3.87

Reheat steam temperature ◦C 541Water feed temperature ◦C 282

2.2. Measurement Port Arrangement and Test Procedure

As illustrated in Figure 2, the vertical measurement ports at SNCR inlets of the threecyclones were all installed in the middle of the roof of the horizontal flue pass and wereabout 700 mm before the injection of the urea spray guns. The sampling probe wascomposed of four 316 L stainless steel tubes with sectional size of 28 mm × 6 mm and alength of 2200 mm; all tubes were connected by threads. With such a long probe, complexchemical reactions could occur at the same time due to the absence of water-cooling, and allthe deviations must be related to the probe length. In our previous study [30], the flue gascould continue to react with C-bearing species of the tube walls, but the S-bearing species

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did not change significantly. Thus, the measurement errors of O2 and CO with a 2.2 m, 316 Lprobe were −4% and 8.6%, and the concentrations of SO2 and NOx had no obvious changes.However, the deviations corresponding to longer probes could not increase linearly to acertain extent as it was also affected by the absolute value of oxygen concentration andgas temperature. In fact, the measurement error of O2 should be obviously higher thanproposed, while the measurement errors of NOx and CO were hard to be clarified atthis stage, as the potential reactions of NO, NO2 and CO with O2 could also cause majorchanges. The authors hoped to reduce the measurement errors within the scope of existingcognition. Thus, the previous obtained measurement errors were adopted, and the samerelative corrections were applied to all the corresponding measured data.

Figure 2. Positions of the vertical measurement ports of the three SNCR inlets: (a) top view; and (b)side view.

The analysis system was composed of the above-mentioned probe, filter device,vacuum pump, three-way valve and gas analyzers. Two gas analyzers were used in thistest, as detailed in Table 2. The Ecom-J2KN type analyzer was for routine use, while theTesto 350 type analyzer would be used in the areas with odd values or large concentrationgradients as a check. According to our experience of on-site tests, the gas compositionwould still show sinusoidal fluctuation under stable loads [32,33], so at least two groups ofpeak/valley values of flue gas should be recorded.

Table 2. Information of the two gas analyzers used in the test.

Item Sensor Type Range Precision (Based on Measured Values)

Ecom-J2KN type analyzer

O2 Electrochemical 0–21% 0.2%CO1 Electrochemical 0–10,000 ppm 5%NO Electrochemical 0–5000 ppm 5%NO2 Electrochemical 0–1000 ppm 5%SO2 Electrochemical 0–5000 ppm 5%

Testo 350 type analyzer

O2 Electrochemical 0–25% 0.8%CO1 Electrochemical 0–10,000 ppm 10%NO Electrochemical 0–4000 ppm 5%NO2 Electrochemical 0–500 ppm 5%SO2

2 Electrochemical 0–5000 ppm 5%1 CO sensors contain H2 compensation. 2 Anti-interference of CO up to 10,000 ppm.

As the height of the horizontal flue pass was 10 m and the ash deposition layer at thebottom was about 2–3 m, the maximum penetration depth of 7 m could basically cover

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the flow area of the SNCR inlets. Meanwhile, to figure out the relationship of flue gascomposition between the SNCR inlets and the dilute phase zone, two measurement portswere installed at 20.6 m above the distributor in the corresponding area of cyclone #A,as shown in Figure 3. The maximum penetration depth of each measurement port wasabout 5 m, so the combustion and pollutant distribution in the whole depth direction ofthe furnace could be obtained. The above two tests were carried out at the same time toensure comparability.

Figure 3. Positions of the horizontal measurement ports in the dilute phase: (a) view from the sidewall; and (b) view from the front wall.

Each horizontal measurement port adopted the urea spray guns at the straight edge ofthe horizontal flue pass, i.e., #1–#6 shown in Figure 4. Because the inner side of the cycloneinlet was inclined, the horizontal measurement could not characterize the cross-sectionaldistribution precisely, so the #7–#10 spray guns were not adopted as measurement ports.The distances between these urea injections and the roof of the horizontal flue pass were1.86, 3.32, 3.9, 6.55, 7.05 and 7.65 m. respectively. The width of the horizontal flue passcorresponding to the spray guns was about 2.2 m, so the sampling probe was composedof two 316 L stainless steel tubes. To avoid the influence of adjacent urea injections, thevalves of the #7–#10 spray guns at the inclined edge were closed in turn during the testinside each spray gun.

Figure 4. Detailed positions of the urea pray guns/horizontal measurement ports at SNCR inlets: (a)view from the straight edge; and (b) view from the inclined edge.

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2.3. Working Condition Arrangement

Three-day stable load periods were applied to keep the unit load and operatingparameters relatively stable, during which the trial test, regular test and repetitive testwere conducted. The main operating conditions corresponding to each measurement arelisted in Tables 3 and 4, where the bed temperatures refer to those in the correspondingarea of each circulating loop. The ultimate and proximate analysis of the experimental coalis shown in Table 5.

Table 3. Main operating parameters of the boiler during the tests of the dilute phase and the verticalmeasurement ports at SNCR inlets (Y-vertical direction)

Item Unit DilutePhase #A-Y #B-Y #C-Y

Load MW 270 270 266 268Coal feed rate t/h 151 151 146 150PA flow rate kNm3/h 507 507 487 499SA flow rate kNm3/h 310 310 299 302Bed pressure kPa 7.41 7.43 7.68 7.69

Bed temperature ◦C 918 920 930 937Cyclone inlet temperature ◦C 932 931 984 975

Cyclone outlet temperature ◦C 882 882 942 933Economizer O2 Vol.-% 3.93 3.93 4.17 4.06

Table 4. Main operating parameters of the boiler during the tests of the horizontal measurementports at SNCR inlets (X-horizontal direction).

Item Unit #A-X #B-X #C-X

Load MW 280 279 264Coal feed rate t/h 170 162 136PA flow rate kNm3/h 532 539 458SA flow rate kNm3/h 319 318 287Bed pressure kPa 8.12 8.21 7.54

Bed temperature ◦C 936 955 933Cyclone inlet temperature ◦C 932 962 952

Cyclone outlet temperature ◦C 898 914 940O2 Vol.-% 4.26 4.15 3.78

Table 5. Proximate and ultimate analysis of the feeding coal during the tests.

Coal Value

Proximate analysis (wt%, air dry basis)Ash 42.33Fixed carbon 49.22Moisture 8.8Volatiles 8.45Lower heating value (MJ/kg) 16.31Ultimate analysis (wt%, air dry basis)C 52.28H 2.69O 1.51N 0.52Stotal 0.67

3. Results and Discussion3.1. Vertical Distributions of Flue Gas Composition at the SNCR Inlets

As mentioned above, to exclude the influence of accidental factors, several tests wereconducted on each vertical or horizontal measurement port of the SNCR inlet. Therefore,

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the typical distribution characteristics with repeatability are presented in the followingresults.

3.1.1. Vertical Distributions of Flue Gas Composition at #A SNCR Inlet

The vertical distributions of flue gas composition at #A horizontal flue pass is exhibitedin Figure 5. During this test, it was confirmed that, when the penetration depth of the probeexceeded 7.5 m, it gradually reached the ash deposition layer. Along vertical direction,the oxygen concentration gradually decreased from 2.7% to 0.37%, and the correspondingCO concentration increased from less than 100 to 1800 mg/m3, indicating that the lowerpart of the SNCR entrance was in an obvious reducing condition. This should be due tothe uneven distribution of fuel and air in the corresponding dense phase zone. On theother hand, the vertical distributions of SO2 and NOx were shown to be parabolic, and thepeak values all appeared at the sampling depth of 5 m. As in-furnace desulfurization wasnot adopted, it is believed that this distribution reflected the actual fuel distribution at thelower part of the furnace in the depth direction.

Figure 5. Vertical distributions of flue gas composition at the #A SNCR inlet.

To confirm this conjecture, Figure 6 illustrates the distribution of flue gas compositionalong the depth direction of the measurement port in dilute phase. It can be seen thatthe distribution trend of O2 and CO concentrations was the same as that in #A SNCRinlet. Higher O2 content was observed near the front wall. while higher CO concentrationappeared near the rear wall. This indicated that the flue gas in the upper part of thehorizontal flue pass mainly came from that in the front wall of the dilute phase zone; thebottom part (except the ash deposition layer) was mainly affected by the flue gas near therear wall; and the middle part should be affected by the flue gas in the central zone of thefurnace. In addition, the NOx concentration along furnace depth also presented a parabolicdistribution with an average value of about 174 mg/m3, while the NOx concentrationat SNCR inlet reached 240 mg/m3 with higher O2 content, indicating that the mixingof gases played a dominant role in promoting further combustion in the upper furnace.Judged from the changes of SO2 distribution, it was believed that combustion was mainlyconcentrated in the central part of the furnace, and the corresponding value increased from6220 to 7937 mg/m3 until the SNCR inlet.

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Figure 6. Horizontal distributions of flue gas composition at the dilute phase zone corresponding tothe #A SNCR inlet.

3.1.2. Vertical Distributions of Flue Gas Composition at #B SNCR Inlet

The vertical distributions of flue gas composition at #B SNCR inlet is exhibited inFigure 7. The O2 content gradually decreased along vertical direction, with the same distri-bution trend as that in the #A SNCR inlet. However, all the measured O2 concentrationswere over 4.69%, and the maximum value was up to 8.88%. Thus, the CO concentrationremained at a very low level. To analyze this combustion difference, the coal feed distri-bution during the test is illustrated in Figure 8. Although some variation was obviouslyunavoidable, it was significant that mass flows were always the highest through feedlines #1 and #8 and lower through the middle ones. This extremely uneven coal feeddistribution must be limited by the primary air distribution along furnace width, so asto provide more uniform bed temperatures. Before the test, the blast caps and secondaryair nozzles were optimized to introduce more uniform air, thus it was reasonable that O2content in the middle part of the furnace was higher. Different from the #A SNCR inlet, thevertical distributions of NOx and SO2 at the #B SNCR inlet showed an increasing trend.Although the corresponding dilute phase zone was not measured, it could be judged fromthe above analysis that the combustion share of the coal should be higher near the rearwall. In this boiler, the coal was introduced into the furnace through seeding coal air, sothe coal distribution over the bed was strongly affected by the initial momentum of fuelparticles. Less coal could be thrown farther under the action of similar seeding air volume.In addition, the average SO2 concentration was only 1772 mg/m3, the strong oxidizingcondition in the middle part could promote the generation and conversion of NOx [34,35].Thus, the average NOx concentration reached 270 mg/m3, which was higher than themeasured value at #A SNCR inlet.

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Figure 7. Vertical distributions of flue gas composition at the #B SNCR inlet.

Figure 8. Fuel distributions during the measurement of at different ports.

3.1.3. Vertical Distributions of Flue Gas Composition at #C SNCR Inlet

The vertical distributions of flue gas composition at #C SNCR inlet is exhibited inFigure 9. The O2 concentration presented a parabolic distribution, and the peak valueappeared at 3 m away from the roof. The distribution of NOx concentration was similar tothat at #A SNCR inlet, and the maximum value also existed at the depth of 5 m. However,the measured value was more than 650 mg/m3, which was far greater than the averageNOx concentration along vertical direction. After repeated tests, the accuracy of thismeasurement result was confirmed. This should be due to the intense combustion offeeding coal in the central part of the furnace corresponding to the #C SNCR inlet, and thisinfluence lasted until the furnace outlet. In any case, it must be related to the initial coaldistribution over the bed in the depth direction. Moreover, the SO2 distribution was highlyconsistent with the NOx distribution, which also confirmed this judgment.

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Figure 9. Vertical distributions of flue gas composition at the #C SNCR inlet.

3.2. Horizontal NOx Distributions at the Cross Section of the SNCR Inlets

It is difficult to describe the distribution characteristics of flue gas composition onlyin the vertical direction of the horizontal flue duct. Therefore, it is necessary to draw acomplete NOx distribution by combining with the horizontal sampling at each height, so asto provide more powerful reference for accurate urea injections. During the measurement,part of the spray guns cannot be inserted due to the obstruction of steel frame, but we triedour best to obtain the NOx distributions of the whole cross sections.

The horizontal distributions of NOx concentration at #A and #C SNCR inlets areexhibited in Figure 10. For the #C SNCR inlet, only the measurements of #5 and #6spray guns were accessible due to the obstruction of the furnace layout. However, threeinteresting phenomena can be summarized: (1) The NOx concentrations in the horizontaldirection of the two SNCR inlets were all maintained within 200–300 mg/m3 except the#3 spray gun of #A SNCR inlet, and the peak value of NOx concentration also appearedjust near the #3 height. (2) No local regions with low NOx concentration due to denitrationreaction were observed. This provided a better prerequisite for optimization of accurateurea injections. (3) For the measured values at the #3 urea spray gun, the NOx distributionhad no changes after repeated tests. Along the horizontal direction, the NOx concentrationfluctuated and suddenly increased from 309 to 2890 mg/m3 after 1200 mm depth, andfinally fell back to about 785 mg/m3 near the inclined wall.

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Figure 10. Horizontal distributions of NOx concentration at the different heights of the #A and #CSNCR inlets.

The horizontal distributions of NOx concentration at #B SNCR inlet is exhibited inFigure 11, where the position with the highest value appeared at the height of #6 ureaspray gun, i.e., the bottom part of the horizontal flue pass. This is also consistent with thedistribution trend shown in Figure 7. Moreover, the NOx concentration at the height of #6urea spray gun increased sharply from 478 to 2258 mg/m3 after 1200 mm depth. In fact, it isdifficult to explain this distribution characteristics, and such a high NOx concentration hasnever been reported in the previous numerical simulations. Therefore, the authors believethat the long-term existence of high NOx concentration in local area should be relatedto the following reasons: (1) local extremely uneven gas–solid flow and the subsequentdevolatilization process during actual boiler operation; (2) the impact and accumulationof particles, together with unstable flow field [36] may occur near the inclined edge of theSNCR inlet, so as to increase the probability of high NOx concentration in this region; and (3)complex chemical reactions in the hot probe, such as oxidation reactions of carbonous dustor hydrocarbon gases and competitive reactions of NO/NO2/CO when O2 was present.This factor was unverified but existed potentially, as, when the probe was immediatelymoved to another measurement depth, the NOx value decreased randomly. After turningback, the NOx value turned to rebound again. These ultra-high concentrations of NOxcould form at the furnace outlet or in the dilute phase zone but were not captured by thistest. For the #C SNCR inlet, only the measurement ports of #5 and #6 spray guns wereaccessible due to the obstruction of the furnace layout. Thus, the corresponding results arenot exhibited in this paper because of the lack of representativeness.

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Figure 11. Horizontal distributions of NOx concentration at the different heights of the #B SNCRinlet.

3.3. Optimization of Urea Injections Based on Uneven NOx Distributions

To save urea consumption, it is necessary to obtain the NOx flux distribution of thecross section. At present, although detailed distributions of NOx concentration at the threeSNCR inlets were obtained, the overall velocity field of flue gas was definitely non-uniform.According to the simulation results of Wang et al. [37], the composition and flow rate hadgreat influence on the urea demand. Therefore, only the preliminary optimization of theflow rate in each urea spray gun was carried out. Before the optimization, the valves of allthe spray gun were full open, and the flow rate of urea solution at each SNCR inlet was thesame. Meanwhile, the NH3 escape detected from the tail flue pass was 8.39 ppm, indicatingthat obvious excessive injection of urea solution was conducted in some local area.

The optimization process was divided into two steps. Firstly, the NOx concentrationswere shown to be the highest in the middle section of #A and #C SNCR inlets based onthe vertical measurement results. Thus, the valve openings of #2, #3, #8 and #9 spray gunswere kept unchanged and the others were gradually reduced. The NOx concentration atthe roof of #B SNCR inlet was the lowest, so the valve opening of #4 and #10 spray gunswere kept unchanged and the others were reduced accordingly. Judged from the horizontalmeasured results, the local area with extremely high NOx concentration just existed at theheight where the overall horizontal NOx concentration was high, so the adjustment of theinner and outer urea spray guns was consistent. Secondly, the flow rate of urea solution inthe main pipe was slowly reduced in the same proportion, while the stack emission of NOxwas detected at the same time. Eventually, the main operating parameters of SNCR processbefore and after the urea optimization are exhibited in Table 6, where the consumption ofurea solution was reduced by 15.7% under the condition that the NOx stack emission wasstill limited below 50 mg/m3.

Table 6. Main operating parameters of SNCR process during urea optimization.

Item Unit BeforeOptimization

AfterOptimization

Urea solution flowrate in #A m3/h 2.800 2.535Urea solution flowrate in #B m3/h 2.694 1.982Urea solution flowrate in #C m3/h 2.601 2.307

NH3 escape ppm 8.39 /Stack NOx emission mg/m3 44.93 47.49

O2 % 6.29 6.06

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It should also be noted that the highest NOx position of #A and #C SNCR inletsappeared at 5 m from the roof, which was just in the middle of #2 (#8) and #3 (#9) sprayguns. Therefore, future work should be focused on the optimization of the position andinjection characteristics of the spray gun and the investigation of initial distribution of fuelparticles over the bed, so as to further improve the NOx-removal efficiency during SNCRprocess.

4. Conclusions

In this study, field tests with deep penetration sampling were conducted on a typicallarge-scale CFB boiler. The horizontal and vertical distributions of flue gas compositionat each SNCR inlet was successfully obtained, and the differences of NOx concentrationwere analyzed in detail combined with air/coal distribution as well as one auxiliary testin dilute phase zone. Although some horizontal ports were inaccessible due to limitationof site layout, the measurements provided a complete picture of the NOx distributioncharacteristics in a large utility CFB boiler for the first time.

The vertical NOx distribution at SNCR inlet was basically consistent with that inthe dilute phase zone, which depended mainly on the initial fuel dispersion along depthdirection. In addition, the vertical NOx concentration at the SNCR inlet on both sidespresented a parabolic distribution but increased along height direction in the middle one.Particularly, some local areas with extremely high NOx concentration (over 2000 mg/m3)were captured near the inclined edge of SNCR inlets, which might be related to the potentialreactions in tubes or local uneven combustion and is worthy of further study.

As a practical conclusion, the preliminary regulation of precise urea injection couldsave 15.7% of urea solution consumption based on the obtained two-dimensional NOxdistributions. Follow-up work should be focused on three aspects: (1) the initial distributionof fuel particles over the bed or even without bed materials; (2) applications of the latestSNCR technologies in large-scale CFB boilers, such as new urea-based ammonia-releasingreduction agents from ERC Technik, Selective Cooling and TWIN-NOx

® technologies fromM&S, acoustic gas temperature measurement and advanced control systems; and (3) morecomprehensive measurements in dilute phase zone to verify their relationship with gasdistribution at the corresponding SNCR inlet, thus to predict the level of original NOxdistribution and the subsequent SNCR process.

Author Contributions: Conceptualization, X.L. and J.Y.; investigation, J.Y., X.Z. and X.F.; methodol-ogy, X.L. and J.Y.; writing—original draft preparation, J.Y.; writing—review and editing, C.Z., Q.L.and J.W.; and validation, S.L. and C.Z. All authors have read and agreed to the published version ofthe manuscript.

Funding: This work was supported by the Scientific Research Start-up Foundation funding of High-level Introduction Talents of Nanjing Institute of Technology (Grants No. YKJ201962), the ProspectiveProject of Industry and University (Grants No. CXY202006) and the National Key Research &Development Program of China (No. 2016YFB0600201).

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: No new data were created or analyzed in this study. Data sharing isnot applicable to this article.

Acknowledgments: The authors are thankful to the stuff from Wu’an Power Plant for valuablesupport during the long-term field tests.

Conflicts of Interest: The authors declare no conflict of interest.

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